Process of and apparatus for making plastic tubing

ABSTRACT

The invention concerns apparatus for molding tube (20) utilizing a travelling mold tunnel. The tunnel is made up of mold blocks (16) each comprising parts (13, 15) which are hinged together to be closeable to form the tunnel and openable to release tube from the tunnel. The opened mold blocks (16) from the downstream end of the tunnel are opened and returned to the upstream end without change in their orientation to reform the tunnel about an extrusion nozzle (18) for thermoplastic extrudate.

This is a continuation-in-part of 07/428,635, filed Oct. 30, 1989, nowabandoned.

This invention relates to apparatus, having travelling mold tunnel forthe production of seamless plastic tubing, especially single ordouble-walled corrugated tubing or ribbed tubing.

BACKGROUND ART

Apparatus of utilizing a travelling mold tunnel is commonly used for theproduction of continuous plastic tubing. The mold tunnel is formedbetween a pair of complementary mold assemblies each of which comprisesan endless run of mold blocks which are articulately interconnected.Each mold block of one mold assembly is complementary to a respectivemold block of the other mold assembly, the mold blocks cooperating inpairs along the forward run to form an axially extending mold tunneldefining a tubular mold. The wall of the mold is dependent on the innerface of the mold blocks and is the matter of choice.

The mold assemblies are driven in synchronism with one another so thatthe mold blocks circulate along the endless tracks. The mold blocks arealways located similarly with respect to the direction of theirmovement. Thus when the direction of movement changes, the orientationof the mold blocks changes with it. For example, the mold blocks on thereturn run are totaled 90° with respect to mold blocks on the tunnelrun. The entrance to the tunnel may be located suitably with respect toan extrusion nozzle for extruding an annular parison of plastic againstthe mold surface of the tunnel. When the resulting tube is required tohave more than one wall, the extrusion nozzle will extrude coaxialparisons of plastic extrudate.

Such apparatus is described in U.S. Pat. No. 3,981,663 to Lupke, issuedSep. 21, 1976 and U.S. Pat. No. 3,832,429 to Charpentier 1974. Thesepatents describe apparatus in which the mold assemblies are arranged oneabove the other. Other conventional apparatus in use is described, forexample in U.S. Pat. Nos. 3,677,676 to Hegler 1972, and 3,751,541 toHaglet 1973. The apparatus of these patents shows the mold assembliesarranged side-by-side. Both these conventional configurations ofapparatus have particular problems associated with them, in that theindependent mold assemblies must be carefully synchronized in operationso that the pairs of cooperating mold blocks come together precisely toform the mold tunnel. Moreover, to provide a pair of independent moldassemblies requires considerable duplication of machinery and equipmentand, in the case of the side-by-side layout, the apparatus covers anappreciable area of floor space which may be an appreciableconsideration where space is limited or expensive.

Any services which must be supplied to the mold blocks must be providedin duplicate for each mold assembly. Such services include the provisionof suction lines, hydraulic fluid lines, possibly heating or coolingservices, etc., maintenance and repair work is also duplicated for themechanism of each endless track. U.S. Pat. No. 4,439,130 issued Mar. 27,1984 to Dickhut discloses the use of mold blocks which are hingedtogether and closed to form the mold tunnel and opened to release formedpairs of tube. This makes possible for a single conveyor but the moldblocks still change orientation with changes in their direction oftravel.

The use of hinged mold blocks which are closed to form the mold tunneland are opened to release a formed tube is disclosed in the DE-B-1 171596. The hinged mold blocks are fixed to a conveyor chain and circulatewith this chain arround sprocket-wheels. During their travel on thereturn path the mold blocks change their direction over an angle of360°.

Since the 1970's various improvements and modifications have been madeto the basic type apparatus disclosed in the patents referred to butexcept for the manufacture of helically corrugated tube no other basictype of apparatus is known. For the production of helically corrugatedpipe, a ribbon of molten thermoplastic has been wound on an advancingmandrel. It is not with this type of apparatus that the presentinvention is concerned.

DISCLOSURE OF THE INVENTION

According to the invention, apparatus for continuous molding ofthermoplastic extrudate e.g. tube, in a travelling mold tunnel to forman elongate molding, comprises; a travelling tunnel mold assembly ofadjacent mold blocks each mold block having a bore therethrough, thebores of adjacent mold blocks being aligned axially; each mold blockcomprising a pair pivotally connected parts whereby the parts aremovable between a closed position of the block in which the bore iscircumferentially enclosed, and an open position in which the parts arelocated with respect to each other release elongate molding formedbetween them; a tunnel conveyor for the tunnel mold assembly to conveyit horizontally and axially in a molding run between tunnel entrance anda tunnel exit; means to open each mold block as it reaches the tunnelexit; means to return each mold block on a return path from the tunnelexit to the tunnel entrance upstream of the return path, and means toclose the returned mold blocks to form the mold tunnel; the mold blockbeing of fixed orientation with its bore parallel to the tunnel axisduring travel on all of the return path to be added to the tunnelentrance. The invention is envisaged for use in the molding ofsemi-rigid plastic tube of the type used for underaround drainage or forcarrying electrical wiring. The cross-section of such tube is generallycircular. For easy exchange of mold blocks for ones of differentdiameter, each mold block may be releasably carried on a mold blockcarrier articulately interconnected mold block carriers.

The means to return each mold block from the tunnel exit to the tunnelentrance may comprise; a removal conveyor arranged to move generally inthe plane of the tunnel conveyor to receive and convey each opened moldblock sequentially from the tunnel conveyor, each mold block beingorientated with its bore parallel to the mold tunnel; a return conveyorgenerally in the plane of the tunnel conveyor arranged to movehorizontally parallel and opposite to the tunnel conveyor to receive andconvey mold blocks sequentially from the removal conveyor, each moldblock being orientated with its bore parallel to the mold tunnel; and anapproach conveyor arranged to move horizontally and parallel to theremoval conveyor, to convey mold blocks sequentially from the returnconveyor, each mold block being orientated with its bore parallel to themold tunnel. Means may be provided to maintain the mold blocks closed onthe tunnel conveyor. Such means may suitably be either guide fences toprevent the mold block parts pivoting away from each other or a camactuated latches between the mold parts.

When the cam actuated latch is provided, the latching cam may be locatedat the upstream end of the tunnel to engage a first cam surface of thelatch to move it into a latching position as the respective mold blockis conveyed on the tunnel conveyor past the cam, and an unlatching cammay be located at the downstream end of the tunnel to engage a secondcam surface of the latch to move it into a latching position as therespective mold block is conveyed or the tunnel conveyor past the cam.In effect, such a latch may be a toggle latch. Once the hinged parts ofthe mold blocks are unlatched, the mold blocks must physically open torelease tube formed in the mold tunnel. This may be achieved throughmold block opening cam means provided at the downstream end of thetunnel, adapted to interact with a cam surface of one mold block part topivot it with respect to the other mold block part to release the tube.Once the tube has been released, the mold blocks may travel on theremoval return and approach conveyors, in either open or closedcondition. However, since the mold blocks must open to receive anextrusion nozzle into the tunnel, it is convenient to maintain the moldblocks open on the removal, return and approach conveyors. At theupstream end of the tunnel, mold blocks pivotally close under gravity.Preferably, such closing is in a controlled manner and mold blockclosing cam means may be provided to interact with a cam surface of eachmold block to close it as it moves into position as part of the moldtunnel.

A change of direction is involved between different conveyors and guidemeans may be provided to guide the mold blocks from one conveyor to thenext. Such guide means may comprise a curved guide rails at outercorners between each conveyor.

The hinging of the mold block parts may be by any convenientarrangement. However, two such arrangements, referred to as exemplary,are envisaged. In both arrangement, the hinge is provided near the topof the mold block, but in one case it is right at the top, and, in theother case, it is offset from the top. In each case, one fixed part ofeach pair of mold block parts is, in use, attached at its base to itsrespective mold block carrier. In the case where the hinge is at the topof the mold block, abutting faces of the block of the fixed and movableparts distant from the hinge, lie in a plane oblique to the radius ofthe bore. By this means it is possible to attach the fixed part over astable base area. Moreover, connection between the fixed and movableparts on the outside of the mold block may be offset from the directbottom, thus allowing access to latching means. In the case where thehinge is offset from the top of the mold block, the fixed and movableparts may be symmetric. The face abutting faces may be radial and offsetfrom the bottom of the mold block diametrically opposite the hinge. Whenthis is so, it may be necessary to lower-the mold block slightly torelease tube from the slightly upstanding edge of the fixes mold block.This may be achieved by making the conveyor run descend slightly. Inthis case, means are provided to lower the mold blocks on the tunnel

A change of direction is involved between different conveyors and guidemeans may be provided to guide the mold blocks from one conveyor to thenext, Such guide means may comprise a curved guide rails at outercorners between each conveyor.

The hinging of the mold block parts may be by any convenientarrangement. However, two such arrangements, referred to as exemplary,are envisaged. In both arrangement, the hinge is provided near the topof the mold block, but in one case it is right at the top, and, in theother case, it is offset from the top. In each case, one fixed part ofeach pair of mold block parts is, in use, attached at its base to itsrespective mold block carrier. In the case where the hinge is at the topof the mold block, abutting faces of the block of the fixed and movableparts distant from the hinge, lie in a plane oblique to the radius ofthe bore. By this means it is possible to attach the fixed part over astable base area. Moreover, connection between the fixed and movableparts on the outside of the mold block may be offset from the directbottom, thus allowing access to latching means. In the case where thehinge is offset from the top of the mold block, the fixed and movableparts may be symmetric. The face abutting faces may be radial and offsetfrom the bottom of the mold block diametrically opposite the hinge. Whenthis is so, it may be necessary to lower the mold block slightly torelease tube from the slightly upstanding edge of the fixed mold block.This may be achieved by making the conveyor run descend slightly. Inthis case, means are provided to lower the mold blocks on the tunnelconveyor to release formed tube from the face of the fixed mold partoffset from the base. In any case, it may be convenient to provide rampsbetween conveyors.

The invention also provides a method for continuous molding ofthermoplastic extrudate in a travelling mold tunnel to form an elongatemolding, comprising: extruding a parison of thermoplastic extrudate intoa travelling tunnel mold assembly of adjacent mold blocks each moldblock having a bore therethrough, the bores of adjacent mold blocksbeing aligned axially; each mold block comprising a pair pivotallyconnected parts whereby the parts are movable between a closed positionof the block in which the bore is circumferentially enclosed, and anopen position in which the parts are located, with respect to eachother, to release elongate molding formed between them; conveying thetunnel mold assembly horizontally and axially in a molding run frombetween a tunnel entrance and a tunnel exit; opening each mold block asit reaches the tunnel exit; returning each mold block on a return pathfrom the tunnel exit to the tunnel entrance upstream of the return path;and closing the returned mold blocks to form the mold tunnel; the moldblock being held in fixed orientation with its bore parallel to thetunnel axis during travel on all of the return path to be added to thetunnel entrance. The method may be used for molding tube. In particular,the mold block bore may be circular in cross-section. Each mold blockmay be carrier on a mold block carrier.

The mold blocks may be returned from the tunnel exit to the tunnelentrance by the following steps: removing opened mold blocks from thetunnel conveyor into a removal conveyor generally running in the planeof the tunnel conveyor, and receiving and conveying each opened moldblock sequentially from the tunnel conveyor, each mold block beingorientated with its bore parallel to the mold tunnel; and transferringthe opened mold blocks from the removal conveyor to a return conveyorgenerally in the plane of the tunnel conveyor running horizontallyparallel and opposite to the tunnel conveyor, and receiving andconveying mold blocks sequentially from the removal conveyor, each moldblock being orientated with its bore parallel to the mold tunnel; andtransferring the mold blocks from the return conveyor to an approachconveyor generally in the plane of the tunnel conveyor runninghorizontally and parallel to the removal conveyor, receiving andconveying mold blocks sequentially from the return conveyor, each moldblock being orientated with its bore parallel to the mold tunnel. Themold blocks closed on the tunnel conveyor may be latched. Furthermorethe mold blocks may be opened at the tunnel exit by a cam channelslidably engaging a lug of each mold block the channel rising to liftthe respective mold block part and thereby open the mold block. The moldblocks may be maintained open on the removal, return and approachconveyors. Alternatively the mold blocks may be opened and closedthrough rotation of a screw threaded spindle acting to adjust the axiallocation of a member in screw threaded engagement therewith, througharticulated linkage with the respective mold block parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the drawings in which:

FIG. 1 is a perspective view of an embodiment of apparatus according tothe invention;

FIG. 2 shows a plan view of the general layout for another embodimentapparatus of the invention;

FIG. 3 is an enlarged view showing one example of a mold block andcarrier which may be used with apparatus according to the invention, forexample as illustrated in FIG. 1;

FIG. 4 is a cross section of the mold block and carrier of FIG. 3 in itsclosed position;

FIG. 5 is a similar cross section of the mold block of FIG. 4 in theopen position;

FIG. 6 is a cross section through another mold block and carrier in theclosed position and suitable for use with apparatus as illustrated inFIG. 1;

FIG. 7 is a cross section through the block of FIG. 6 in the openposition;

FIG. 8 is a view of carriers, suitable for use in apparatus asillustrated in FIG. 2;

FIG. 9 is a view of another closed mold block in combination with acarrier as shown in FIG. 8 indicating mold opening and closingmechanism; and

FIG. 10 is a view of the mold block of FIG. 9 in the open position.

MODES OF CARRYING OUT THE INVENTION

In the drawings, FIG. 1 shows a general view of apparatus embodying theinvention which may comprise a tunnel conveyor 12 along which travels atunnel mold assembly 14, segments of which are hinged mold blocks 16 ofwhich are closed in the tunnel mold assembly over the majority of tunnelrun of a conveyor 12. Each mold block 16 comprises a fixed half 13 and ahinged half 15 and each has a carrier block 17 which may be driven tomove along the conveyor 12 in a manner to be described hereafter. At anupstream end of the tunnel assembly 14, an extrusion nozzle 18 entersthe tunnel to extrude an annular parison 19 of thermoplastic material toform tube 20 within the tunnel. At the downstream end of the tunnelassembly, tube 20 emerges from the tunnel. Also at the downstream end ofthe tunnel, mold blocks 16 open hingeably to release tube 20 and arereturned to the upstream end of the tunnel conveyor 12 by means of aremoval conveyor 22, a return conveyor 24 and an approach conveyor 26.Conveniently, the tunnel conveyor 12 and the removal conveyor 22, thereturn conveyor 24 and the approach conveyor 26 are laid out in thegeneral form of a rectangle, the upper runs of each conveyor being atleast generally in the same horizontal plane. While the speeds of thevarious conveyors may be coordinated to return mold blocks to the tunnelconveyor at a suitable speed to form the tunnel assembly 14 on thetunnel conveyor, in a practical embodiment the tunnel conveyor comprisesa stationary conveyor along which the mold blocks 16 are driven viatheir carriers 17.

On the removal, return and approach conveyors 22, 24 and 26 the moldblocks may travel in either an open or closed condition. However, sincethe mold blocks-must open to release tube 20, and must be open in orderto close around extrusion nozzle 18, it may normally be convenient forthem to travel on at least most of the route back to the upstream end ofthe tunnel conveyor 12 in the open condition. Mold blocks halves mayopen as much as 180° to each other or even more. However, it is normallyconvenient that they only open widely enough to release tube 20 frombetwen them.

When assembled into mold tunnel assembly 14 on tunnel conveyor 12, it isimportant that the mold blocks 16 have upstream faces and downstreamfaces 11 in contact with one another to form a closed tunnel in thelongitudinal direction. It may normally be sufficient to press the moldblocks firmly together on this run but it may be advantageous tomechanically link the carriers, as for instance, by the illustratedtongue 100 and groove 102 connections between them as seen from FIG. 8.The halves 13, 15 of each mold block 16 must, on the tunnel run tightlyclosed about the hinge 28 so that the tunnel is also tightly closedaround its circumference, and must reliably open to release tube 20.This may be achieved by various means, for example the cam arrangement50, 5a shown in FIG. 1 or the mechanism 104 shown in FIGS. 9 and 10.

The alignment of the mold blocks 16 on the tunnel conveyor 12 is alsoimportant so that the extrusion nozzle 18 is accurately centered in thetunnel for accuracy in molding the tube 20. Conveniently, a track 30 isprovided on tunnel conveyor 12 for engagement with a corresponding part31 of each carrier 17 for guidance and location of mold block 16. Thetrack 30 may be in the form of an upstanding rail to cooperate with agroove 31 shaped slot 31 of each mold block. The rail 30 may house asource of suction and suction passages 33 through carrier 17 may connectsuction ports 35 of the rail 30 with suction passages 37 in the moldblock 16 leading to its interior. The provision of suction to theinterior of the tunnel may be provided in a similar manner to thatdescribed in any of U.S. Pat. No. 4,319,872 or Canadian patent No.1,083,765, these patents are directed to a travelling mold comprisingmold blocks through which suction may be applied, although all of thepatents are limited to the use of two conveyor systems, one for each oftwo runs of half mold blocks. Nevertheless, the provisions of suctionmeans within guides for the mold blocks may be regarded as similar.

Suitably the carriers 17 "wrap around" the track 30 at its edges, oraround an additional edge track 39, by means of an inwardly extendingflange 41, to prevent accidental disengagement of the carriers 17 andblocks 16. The length of the tunnel conveyor and the length of thetunnel assembly 14 should be sufficient for extruded annularthermoplastic parison to set into tube 20 of sufficient rigidity for theremoval of mold blocks 16 without permanent distortion of the tube.Cooling plugs, and other forming means for the inside surface of thetube may be provided in a conventional manner supported on an inner coreextending coaxially from the extrusion nozzle. For simplicity, suchdevices are not illustrated. However, additional cooling means ispossible on the outer surface of the mold, as for example, by ribs 106on its outer surface as shown in FIG. 9. Cooling water may be flowedover the exterior of the closed mold tunnel assembly to aid cooling anddrain channels 108 may be provided for the water.

When the mold assembly has remained closed over a sufficient length forthe tube to set, the mold blocks may be opened by hinging each of themabout its hinge 28. Once open, the blocks may be removed from the tube,for example, orthogonally therefrom along removal conveyor 22. Thus theface of the mold block which was the downstream face of the tunnel runis now a side face since the orientation of the mold block remainsfixed. While, as already commented, the speeds of the various conveyorsshould be coordinated, there is no particular reason for accuratealignment of the mold blocks 16 in tunnel formation on any of theremoval conveyor 22, the return conveyor or the approach conveyor.

In practice, it is convenient that removal conveyor 22 runs at an angleof slightly more than 90° to the tunnel conveyor and that mold blocksare located on it with their tunnel axis not quite aligned with thedirectional axis of conveyor 22. Thus, the orientation of mold blocks 16is the same as on tunnel conveyor 12 so that they are set at a slightangle on conveyor 22. On tunnel 12 mold blocks 16 more in the directionof the tunnel axis. Whereas, on conveyor 27 they move in a direction atslightly greater than 90° to the tunnel axis of each block 16.

It is convenient to remove the mold blocks 16 in a direction parallelwith the upstream faces and the downstream faces 17 and, althoughsufficient guidance means should be provided to prevent them falling offthe conveyor, there is no need for accurate guide means such as track30. The guidance means may conveniently be chains 110. The provision ofa guide rail 32 extending alongside removal conveyor 22 and, at leastpartially, alongside tunnel conveyor 12 may be useful in some instances.

When a tongue 100 and groove 102 linkage is provided between mold blockscarriers 17 (See FIG. 8) on the tunnel run, this linkage must bedisconnected to allow the carriers 17 with their associated blocks 16 totravel essentially edge to edge on the removal run rather than face toface.

The linkage may comprise a rail of T-shaped cross section or, asillustrated an aligned pair of lugs 100 of T-shaped cross section on onetunnel face of a carrier, e.g. the downstream face 11 and acorresponding T-shaped groove 102 in the other face. When travelling ina tunnel direction the T-groove 102 interlocks the T-rail 100 to holdfaces of adjacent blocks in register. However, when a block 16 and itsassociated carrier 17 are subject to an approximate 90 degree change ofdirection without change of orientation, as indicated, for example, bythe arrow A in FIG. 8, (or in the 180 degree direction), then T-members100 will slide out of T-grooves 102 to unlatch the carriers and allowthem to move away in the new direction.

The movement of carriers 17 (and hence blocks 16) on the tunnel conveyormay be by means of driven sprocket wheels 32 which mesh with lugs 112provided on one, or as illustrated both, sides of carriers 17. Suchsprocket wheels 32 at the upstream of the tunnel run may convenientlydrive the entire tunnel train of carriers 17 and mold blocks 16.

Movement of the carriers and mold blocks on removal 22, return 24, andapproach 26 conveyors may be by any convenient means. However, it is tobe noted that similar sprocket wheels may be used or the return run andchains 110 which may run on sprocket wheels 114 driven in any convenientmanner. For additional ease in transition from one direction to another,where one conveyor adjoins another conveyor travelling in a differentdirection, ramps may be provided in the conveyors.

The position of hinge 28 and the comparative size of the mold blockparts which are hinged together about the hinge are a matter of choice.However, it is found that the apparatus may offer advantages for themolding of large diameter tube. For example, tube diameter greater than12" is moldable using the method and apparatus according to theinvention.

As shown in FIGS. 1, 3, 4, 5, 9 and 10 the hinge of each mold block 16is located at the top of a mold block 16 and connects mold block parts13, 15 each of which comprises roughly half the mold block 16 althoughany convenient proportion may be chosen.

When hinge 28 is at the top of the mold block, it is preferable to shapethe fixed part 13 (conveniently referred to as halves), fixed half 13,so that it includes the whole of the base of the mold block whichconnects with carrier 17 in any conventional manner. Thus the guideblocks may suitably have the shape illustrated in FIGS. 3, 4 and 5.These guide blocks have faces 42, 43 adjoining the hinge 28 which areflush with each other when the guide block is closed to lie radially ofthe mold tunnel. These blocks also have faces 44, 45 which are flushwith each other when the guide block is closed and spaced apart from oneanother when the guide block is open. These faces, when the guide blockis closed, lie in a plane which is oblique to the radius of the tunnel.

A guide block as shaped from two asymetric sectors as shown in FIGS. 3,4 and 5 is advantageous in some respects in that it is easy to removefrom the formed tube 20. As shown, with the hinge at the top most pointof the block and the opening of the tunnel at the lower most point ofthe tunnel, the block is easily withdrawable from the tube in adirection orthogonal to it, for example, on the removal conveyor 22.When the block has this shape, there is no interference or dragging onthe tube 20 when the opened block 16 is pulled away from it.

Another desirable shape of mold block is shown in FIGS. 6 and 7. In thiscase, the two halves 13, 14 of the block 16 are symmetric and thus haveadvantages in manufacture. The hinge 28 is offset from the top of themold block and fixed half 13 is attached carrier 17. In this case, bothpairs of faces 42, 43 and 44, 45 are radial to the tunnel when the moldblock is closed. However, because the hinge 28 is offset from theuppermost point of the mold block, the lower opening point of the moldblock is not coincident with the lowermost point of the tunnel. Thuswhen an attempt is made to pull the mold block away from formed tube 20,it will tend to drag the tube 20 with it if is pulled in a strictlyhorizontal direction. In this case, it is desirable to adjust the levelof removal conveyor 22 to be slightly lower than that of tunnel conveyor21. The difference in height between tunnel conveyor 12 and removalconveyor 22 may be the same as the difference in height between thelower opening point 46 of the mold block and the lowest point of thetunnel 47. The conveyors 12, 22, 24 and 26 may all be in the samegenerally horizontal plane. Any slight reduction in height of conveyor22 is being very slight and may be regarded as being within the samegeneral horizontal plane as the upper runs of the other conveyors.

From conveyor 22 the mold blocks 16 and carriers 17 are transferred toreturn conveyor 24 which may, as shown, run parallel and opposite to thedirection of tunnel conveyor 12. On return conveyor 24, the mold blocks16 and carriers 17 are again travelling in the direction of the tunnelaxis and will probably, due to the speed be close together in tunnelformation and, again linked through T-bars 100 and T-grooves 102. Thereis, however, no theoretical need to ensure that they fit tightlytogether with their upstream and downstream faces in tight flush fit.Nor is there a need to provide for accurate alignment of the tunnel.Therefore, for conveyor 24, there is no need to provide a track similarto track 30. The faces of the mold block which were downstream faces onthe tunnel run are now upstream faces because the orientation of themold blocks is fixed.

At the end of conveyor 24 the mold blocks change direction again ontoapproach conveyor 26 in a similar manner to the changes of directionbefore described. Now the faces of the mold blocks which were downstreamfaces on the tunnel run are, again, side faces because the orientationof the mold blocks is fixed. When on approach conveyor 26, at least whenapproaching extrusion nozzle 18, it is important that the mold blocks bein open position. It is possible to start closing the mold blocks beforethey reach extrusion nozzle 18, provided there is sufficient clearancebetween the hinged half 14 and the extrusion nozzle as the mold movesinto position for the start of another mold tunnel run.

As the open mold block 16 approaches the beginning of the mold tunnelconveyor 12 and the extrusion nozzle 18, it may be closed by anyconvenient means.

One means of opening and closing the mold blocks 16 is shown in FIG. 1.A cam track 50 may be provided following the general layout of conveyors12, 22, 24, 26. Cam track may be of C-section to accept and retain theheads of 60 mushroom lugs 52 (exteriors) outwardly from hinged half 14of each mold block. Along the tunnel conveyor 12, a continuous cam track50 opens inwardly and is located at a height lateral of the conveyor 12,as to maintain the mold blocks closed. As the downstream end of conveyor12 approaches, cam track 50 rises to hingedly raise each mold half 14and open the mold block 16. The path of cam track 50 follows the trackof the leads 60 of the mushroom lugs 52 and, as the mold blocks 16changes direction onto the removal conveyor, twists so that it opensdownwardly. The cam track 50 may rise to hingedly raise the mold half 18through 180° but this is not necessary. It may be easier to only openthe mold block 16 sufficiently for removal of tube 20. The cam track 50may be maintained at this height over the removal conveyor 22, thereturn conveyor 24 and the approach conveyor 26. At the downstream endof the approach conveyor 26, the cam track descends to close the moldblock 16 and twists to open again towards the tunnel conveyor 12.

A preferred manner of opening and closing the mold blocks 16 isillustrated in FIGS. 8, 9, and 10, which illustrate a mold block 16 anda carrier 17 having mold opening mechanism carried by the carrier. Thismold opening mechanism comprises screw threaded vertical spindle 34carried in an upstanding framework side 36 of carrier 17. At its lowerend spindle 34 carries sprocket wheel 38 geared to actuating sprocketwheel 40.

Actuating sprocket wheel 40 may be powered by any convenient means suchas an electric motor or other motor. Alternatively, at least on thetunnel run, a rack 46 may be provided so that motion of the carrierspast the rack 46 will engage wheel 40 or 38 to turn spindle 34.

Turning of spindle 34 in the direction shown by arrow-B in FIG. 9 willlower block 48 engaged on the spindle 34. Block 48 has an axle 62projecting to either side of it carrying one end of an articulated arm64, comprising a member 65 hinged to hinged mold block half 14, a member66 hinged to fixed mold block half 13, a rigid link 67 pivotallyconnected at its respective ends to members 65 and 66, and a member 68pivotally connected at one end to member 66 and link 678 and, at theother end to block 48.

As block-48 descends spindle 34 the action of articulated arm 64 is toopen mold block as shown in FIG. 10.

The outer surface of mold blocks 16 may have cooling ribs 70.

I claim:
 1. Apparatus for continuous molding of thermoplastic extrudatein a travelling mold tunnel to form an elongate molding, comprising:atravelling tunnel mold assembly of adjacent mold blocks each mold blockhaving a bore therethrough, the bores of adjacent mold blocks beingaligned axially; each mold block comprising a pair of pivotallyconnected parts whereby the parts are movable between a closed positionof each mold block comprising a pair of pivotally connected partswhereby the parts are movable between a closed position of the block inwhich the bore is circumferentially enclosed, and an open position inwhich the parts are located with respect to each other to releaseelongate molding formed between them; a tunnel conveyor for the tunnelmold assembly to convey it horizontally and axially in a molding runbetween a tunnel entrance and a tunnel exit; means to open each moldblock as it reaches the tunnel exit; means to return each mold block ona return path from the tunnel exit to the tunnel entrance, upstream ofthe return path, said means to return including removal, return, andapproach conveyors each lying generally in a horizontal plane common tothat of the tunnel conveyor and adapted to communicate each mold blockfrom the tunnel exit to the tunnel entrance while each mold block isoriented with its bore parallel to the tunnel axis during travel on thereturn path; and means to close the returned mold blocks to form themold tunnel.
 2. Apparatus as claimed in claim 1, in which the mold blockbore is circular in cross-section.
 3. Apparatus as claimed in claim 2,in which each mold block (16) is carried on a mold block carrier (17).4. Apparatus for continuous molding of thermoplastic extrudate in atravelling mold tunnel to form an elongate molding, comprising:atravelling tunnel mold assembly of adjacent mold blocks, each mold blockbeing carried on a mold block carrier, and each mold block having a boretherethrough, the bores of adjacent mold blocks being aligned axiallyand having a circular cross-section; each mold block comprising a pairof pivotally connected parts where the connected parts are movablebetween a closed position of the block in which the bore iscircumferentially enclosed and an open position in which the connectedparts are located with respect to each other to release elongate moldingformed between them; a tunnel conveyor for the tunnel mold assembly toconvex it horizontally and axially in a molding run between a tunnelentrance and a tunnel exit; means to open each mold block as it reachesthe tunnel exit; means to return each mold block on a return path fromthe tunnel exit to the tunnel entrance, upstream of the return path, andmeans to close the returned mold blocks to form the mold tunnel, saidmeans for returning including: a removal conveyor arranged to movegenerally in the plane of the tunnel conveyor to receive and convey eachopened mold block sequentially from the tunnel conveyor, each mold blockbeing orientated with its bore parallel to the mold tunnel; a returnconveyor generally in the plane of the tunnel conveyor arranged to movehorizontally parallel and opposite to the tunnel conveyor to receive andconvey mold blocks sequentially from the removal conveyor, each moldblocks being orientated with its bore parallel to the mold tunnel; andan approach conveyor generally in the plane of the tunnel conveyorarranged to move horizontally and parallel to the removal conveyor, toconvey mold blocks sequentially from the return conveyor, each moldblock being orientated with its bore parallel to the mold tunnel, themold block being of fixed orientation with its bore parallel to thetunnel axis during travel on all of the return path to be added to thetunnel entrance.
 5. Apparatus as claimed in claim 4 in which means areprovided to maintain the mold blocks (16) closed on the tunnel conveyor(12).
 6. Apparatus as claimed in claim 5 in which the means comprises acam actuated latch (50, 52) between the mold parts.
 7. Apparatus asclaimed in claim 5 in which the cam actuated to maintain the mold blocks(76) closed comprises a cam channel (50) slidably engaging a lug (52) ofeach mold block (16).
 8. Apparatus as claimed in claim 7 in which thecam channel (50) is continuous and is located at a height above theconveyors (12, 22, 24, 26) to maintain mold blocks (16) closed duringtravel along the tunnel conveyor (12) and open on the removal, returnand approach conveyors (22, 24, 26).
 9. Apparatus as claimed in claim 4in which the means to open the mold block (16) and the means to closethe mold blocks (104) comprises a rotatable screw threaded spindle (34)on each mold block carrier (17), a member (48) screw threadedly engagingthe spindle (34) for axial adjustment of its location thereon byrotation of the spindle (34), and articulated linkage (64, 75, 66, 67,68) between said member (48) and the respective mold block (16)pivotally connected parts (13, 15) for opening and closing of the moldblocks (16) a dependance on the axial location of said member (48). 10.Apparatus as claimed in claim 9 in which the spindle (34) is driven inan opening direction for the mold blocks (16) by a gear wheel (40)engaging a rack (46) located on the tunnel conveyor (12).
 11. Apparatusas claimed in claim 4, in which one fixed part (13) of each pair of moldblock parts (13, 15) is, in use, attached at its base to its respectivemold block carrier (17), and is provided with a hinge (28) at the top ofthe mold block for pivotal connection with the other movable part (15)of said pair.
 12. Apparatus as claimed in claim 11 in which abuttingfaces (44, 45) of the fixed and movable mold block parts (13, 15)distant from the hinge (28) lie in a plane oblique to the radius of thebore.
 13. Apparatus as claimed in claim 4 in which one fixed part (13)of each pair of mold blocks (16) in use, attached at an attachment pointof its respective carrier, and is provided with a hinge (28) offset fromthe top of the mold block (16) for pivotal connection with the othermovable part (13) of said pair.
 14. Apparatus as claimed in claim 13 inwhich abutting faces (44, 45) distant from the hinge (28) of the fixedpart (13) and the movable part (15) are offset from the base of the moldblock and radially arranged and diametric with hinge faces (42, 43) ofthe fixed and movable parts (13, 15).
 15. Apparatus as claimed in claim14 in which, at the downstream end of the tunnel and downstream of themeans to open each mold block (16), means are provided to lower the moldblocks on the tunnel conveyor to release formed tube (20) from the face(44) of the fixed mold part offset from the base.
 16. Apparatus asclaimed in claim 14 in which the tunnel conveyor comprises a fixed guide(30, 39) and drive means (32, 112) are provided to drive mold blocks(16) to move over the guide (30, 39).
 17. Apparatus as claimed in claim15 in which the drive means comprise a drive sprocket wheel (32)sequentially engaging drive tugs (112) projecting from each mold blockcarrier (17).
 18. Apparatus as claimed in claim 4 in which engagementmeans (100, 102) are provided for sliding engagement of adjacent moldblocks (16) on the tunnel conveyor (12).
 19. Apparatus as claimed inclaim 18 in which the engagement means comprises for each mold block, agroove (102) along one of a leading edge and a tracking edge of eachmold block carrier (17) and a tongue (100) adapted to make with thegroove (102) of the other of the leading and tracking edges.
 20. Amethod for continuous molding of thermoplastic extrudate in a travellingmold tunnel to form an elongate molding, comprising the stepsof;extruding a parison of thermoplastic extrudate into a travellingtunnel mold assembly of adjacent hinged mold blocks each mold blockbeing pivotable between the closed mold tunnel forming position and anopen position having a bore therethrough, the bore of adjacent moldblocks being aligned axially, conveying the tunnel mold assembly in ahorizontal plane and axially in a molding run between a tunnel entranceand a tunnel exit; opening each mold block as it reaches the tunnelexit; returning each mold block along a substantially horizontal returnpath which is generally coplanar with the molding run from the tunnelexit to the tunnel entrance upstream of the return path, and closing thereturned mold blocks to form the mold tunnel.
 21. A method as claimed inclaim 20, for molding tube (20).
 22. A method as claimed in claim 21, inwhich the mold block bore is circular in cross-section.
 23. A method asclaimed in claim 22, in which each mold block (16) is carried on a moldblock carrier (17).
 24. A method for continuous molding of thermoplasticextrudate in a travelling mold tunnel to form an elongate molding,comprising;extruding a parison of thermoplastic extrudate into atravelling tunnel mold assembly of adjacent mold blocks each mold blockhaving a bore therethrough, the bore of adjacent mold blocks beingaligned axially, each mold block comprising a pair pivotally connectedparts whereby the parts are movable between a closed position of theblock in which the bore is peripherally enclosed, and an open positionin which the parts are located, with respect to each other, to releasethe elongate molding formed between them; conveying the tunnel moldassembly horizontally and axially in a molding run between a tunnelentrance and a tunnel exit; opening each mold block proximate to thetunnel exit; returning each mold block on a return path from the tunnelexit to the tunnel entrance upstream of the return path by removingopened mold blocks from the tunnel conveyor onto a removal conveyorgenerally running in the plane of the tunnel conveyor, and receiving andconveying each opened mold block sequentially from the tunnel conveyor,each mold block being orientated with its bore parallel to the moldtunnel; and transferring the opened mold blocks from the removalconveyor to a return conveyor generally in the plane of the tunnelconveyor running horizontally parallel and opposite to the tunnelconveyor, and receiving and conveying mold blocks sequentially from theremoval conveyor, each mold block being orientated with its boreparallel to the mold tunnel; and transferring the mold blocks from thereturn conveyor to an approach conveyor generally in the plane of thetunnel conveyor running horizontally and parallel to the removalconveyor, receiving and conveying mold blocks sequentially from thereturn conveyor, each mold block being orientated with its bore parallelto the mold tunnel; maintaining the mold blocks to be added to thetunnel entrance in a fixed orientation with their bore parallel to thetunnel axis; and closing the returned mold blocks proximate to thetunnel entrance to form the mold tunnel.
 25. A method as claimed inclaim 24, including latching the mold blocks (16) closed on the tunnelconveyor (12).
 26. A method as claimed in claim 25, in which the moldblocks (16) are opened at the tunnel exit by a cam channel (50) slidablyengaging a lug (52) of each mold block (16) the channel (50) rising tolift the respective mold block part (15) and thereby open the mold block(16).
 27. A method as claimed in claim 26, in which the mold blocks (16)are maintained open on the removal, return and approach conveyors (22,24, 26).
 28. A method as claimed in claim 24 in which mold blocks (16)are opened and closed through rotation of a screw threaded spindle (34)acting to adjust the axial location of a member (48) in screw threadedengagement therewith, through articulated linkage (64, 65, 67, 68) withthe respective mold block parts (13, 15).
 29. Apparatus for continuousmolding of thermoplastic extrudate in a travelling mold tunnel to forman elongate molding, comprising:a travelling tunnel mold assembly ofalignable, pivotally hinged, mold blocks each having an axiallyalignable bore therethrough and each mold block being pivotal between aclosed tunnel-forming position and an open position and each mold blockbeing drivable connected to a motor for opening and closing the moldblock; a generally planar conveyor for conveying the mold blocks in apathway generally lying in a common plane where the pathway includes anaxially extending molding run between a tunnel entrance and a tunnelexit, a lateral removal run to move an individual mold block away fromthe tunnel exit, a return run generally parallel to the axial moldingrun, and an approach run for guiding the mold block to the tunnelentrance; and means to close the mold block at the tunnel entrance toform the mold tunnel.
 30. A method for continuous molding ofthermoplastic extrudate in a travelling mold tunnel to form an elongatemolding, comprising;extruding a parison of thermoplastic extrudate intoa travelling tunnel mold assembly of adjacent mold blocks each moldblock having a bore therethrough, the bore of adjacent mold blocks beingaligned axially, each mold block comprising a pair pivotally connectedmating sections which are movable between a closed position and an openposition; conveying the tunnel mold assembly horizontally and axially ina molding run between a tunnel entrance and a tunnel exit; opening eachmold block proximate to the tunnel exit; establishing a generallyhorizontally disposed return path co-planar with the molding run, thereturn path being defined by a removal run, a return run, and anapproach run where the removal and approach runs are generally parallelto each other and both are generally perpendicular to the return run;and returning each mold block on the return path from the tunnel exit tothe tunnel entrance upstream of the return path.